associate ss3diags with the "stock-synthesis" topic

[iantaylor-NOAA]

@k-doering-NOAA and I just learned about topics on github and associated a bunch of Stock-Synthesis-related repositories with the "stock-synthesis" topic (https://github.com/topics/stock-synthesis).

You can use these instructions: https://docs.github.com/en/github/administering-a-repository/classifying-your-repository-with-topics to add ss3diags to that list which would improve its visibility among SS users.

Speaking of visibility, you should update the reference on the README with a link to the cookbook paper.

[iantaylor-NOAA]

The PIFSC version of ss3diags (https://github.com/PIFSCstockassessments/ss3diags) is associated with the "stock-synthesis" topic. Thank you!